This invention relates to the nucleation of ice in the freezing of foodstuffs and like biological materials and additives for use in promoting such nucleation.
It is important in the freezing of foodstuffs and like biological materials to minimize the formation of large ice crystals since this has a generally deleterious effect on the texture or condition of the material after thawing. It is found that if freezing is effected in the presence of a large number of nuclei promoting ice formation, all the water present can be frozen in the form of small crystals. However, there are serious limitations on the substances which can be added to food, so that nucleating additives have not hitherto been used to a significant extent.
It has recently been proposed to add to foodstuffs cells of Pseudomonas syringiae which carry a native protein capable of ice nucleation. This organism is widely distributed and is responsible in many substances for premature freezing of growing plants such as grape vines, strawberry, potato, tomato plants and the like. It is found that cells of P. syringiae can promote ice nucleation and the formation of small ice crystals when present in foodstuffs during freezing. In addition, U.S. Pat. No. 4,200,228 describes a method using such organisms in snow making while International Patent Application WO 83/03831 describes the transformation of certain host microorganisms with DNA coding for the ice nucleation protein in order to produce microbial agents of use in inhibiting supercooling.
However, although yeasts and other fungi have been considered as possible hosts in the above methods, only E. coli and the native organisms such as P. syringiae are specifically mentioned. There is thus no specific mention of any host microorganisms which could be used practically as additives to biological materials for ingestion such as foods.
P. syringiae is a member of the genus Pseudomonas. Certain species of the genus are pathogens, for example the human pathogen P. aeruginosa. Some strains of P. syringiae are also known to be pathogens for certain plants. Similarly, certain strains of E. coli are well known human pathogens. Consequently, they would not be practicable for use in food additives. Even when used in the form of killed cells, there is always the danger of incomplete killing and subsequent proliferation of surviving organisms on storage and thawing of the foodstuff and such microorganisms are not included in the lists of organisms designated GRAS (Generally Regarded as Safe) in the U.S. Code of Federal Regulations, Vol. 21, Parts 170 to 199, April 1986, published by the U.S. Government Printing Office.